.alpha.-alumina powder is widely used as raw materials for abrasives, sinters, plasma spraying materials, fillers, etc. .alpha.-Alumina powder obtained by conventional processes that have been generally employed comprises irregular-shaped polycrystals, contains many agglomerated particles, and has broad particle size distribution. The purity of such conventional .alpha.-alumina products is insufficient for some uses. In order to avoid these disadvantages and, for some specific uses, to control primary particle size or shape, .alpha.-alumina powder produced by special processes as hereinafter described has been employed. However, these special processes still involve difficulty in producing .alpha.-alumina powder with a narrow primary particle size distribution, which comprises .alpha.-alumina particles having a controlled shape, a controlled primary size, and homogeneity. Further, in order to orientate .alpha.-alumina particles in a specific direction when packed or laminated in layer(s), it is desirable that the powder comprises .alpha.-alumina particles having a rod-like shape, a cocoon shape or a thick plate shape. However, conventionally obtained .alpha.-alumina powder has broad primary particle size-distribution or the particles thereof have a thin plate shape, and it has hitherto been difficult to obtain .alpha.-alumina powder having a narrow primary particle size distribution and the particles thereof having such a shape suitable for orientation.
Among general processes for producing .alpha.-alumina powder a Bayer's process is the most economical process. In a Bayer's process, bauxite is once converted to aluminum hydroxide or transition alumina, which is then calcined in air to prepare .alpha.-alumina powder.
The aluminum hydroxide or transition alumina which is obtained as an intermediate product on an industrial scale at low cost comprises agglomerated particles having a diameter of greater than 10 .mu.m. .alpha.-Alumina powder obtained by calcination of such aluminum hydroxide or transition alumina in air comprises primary particles of irregular shape containing many coarse particles agglomerated strongly. The .alpha.-alumina powder containing coarse agglomerated particles are ground into final products by means of a ball mill, a vibration mill, etc., but grinding is not always easy and incurs the cost. Further, .alpha.-alumina powder having poor grindability needs a long grinding time, consequently too fine powder may be formed.
Several proposals have been made to solve these problems. For example, JP-A-59-97528 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") teaches a process for improving the shape of .alpha.-alumina powder, which comprises calcining aluminum hydroxide prepared by a Bayer process in the presence of an ammonium-containing boron or boron series mineralizer to obtain .alpha.-alumina powder having an average primary particle diameter of from 1 to 10 .mu.m and a D/H ratio approximate to 1, wherein D is a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of .alpha.-alumina, and H represents a maximum particle diameter perpendicular to the hexagonal lattice plane. Since the starting aluminum hydroxide has a particle diameter of several tens of micrometers (.mu.m) or greater, and the calcination is carried out in a rotary kiln, the resulting .alpha.-alumina powder has a broad primary particle size distribution and the particles thereof have irregular shapes. It is also difficult to arbitrarily control the primary particle size or shape.
Known special processes for producing .alpha.-alumina powder include a hydrothermal process utilizing a hydrothermal reaction of aluminum hydroxide; a flux process comprising adding a flux to aluminum hydroxide, fusing, and precipitating; and a process comprising calcination of aluminum hydroxide in the presence of a mineralizer.
With respect to a hydrothermal process, JP-B-57-22886 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses addition of corundum as a seed crystal to control the particle size. Because the synthesis in this process is carried out in a high temperature under a high pressure, it involves a problem in that the resulting .alpha.-alumina powder becomes expensive.
A flux process has been proposed as a means for controlling the particle shape or primary particle size of .alpha.-alumina powder for use as an abrasive, a filler, etc. For example, JP-B-3-131517 discloses a process comprising calcining aluminum hydroxide in the presence of a fluorine series flux having a melting point of not more than 800.degree. C. to prepare .alpha.-alumina powder which comprises hexagonal plate-shaped .alpha.-alumina particles having an average primary particle size of from 2 to 20 .mu.m and a D/H ratio of from 5 to 40, wherein D and H are as defined above. However, this process cannot provide fine .alpha.-alumina powder having a primary particle diameter of 2 .mu.m or less, and all the particles obtained have a plate shape. In other words, the process was unable to arbitrarily control the shape and particle size.
Journal of American Ceramic Society, Vol. 68, No. 9, pp. 500-505 (1985) reports that the temperature of a transition can be reduced by addition of .alpha.-alumina to boehmite. However, since the purpose thereof is to obtain a sintered body of fine grain size, .alpha.-alumina powder with controlled primary particle size and shape cannot be obtained by this technique.
U.S. Pat. No. 4,657,754 discloses a process for obtaining .alpha.-alumina powder whose particles are smaller than 1 .mu.m in diameter comprising adding .alpha.-alumina seed crystals to an .alpha.-alumina precursor, followed by calcination and grinding. The powder obtained by the above calcination comprises agglomerates of fine primary particles of not greater than 1 .mu.m, and the primary particles greater than 10 .mu.m cannot be obtained by the process.
Therefore, there has not yet been established a technique for producing .alpha.-alumina powder in which the primary particle diameter can arbitrarily be controlled from submicrons to several tens of microns; the particle shape can be controlled from a hexagonal plate shape to a column shape; or the particle size distribution can be narrowed while arbitrarily controlling the crystal habit of the a face {1120}, c face {0001}, n face {2243} and r face {1012}. There has been a keen demand to develop such techniques.
Moreover, .alpha.-alumina powder whose particles have a sufficient thickness for easy orientation, has a narrow primary particle size distribution, and is particularly suitable as a raw material for abrasives, fillers, sinters or spacer has not yet been obtained.